Schroeder, Faculty of Geology and Graduate Council Representative for his suggestions and guidance in the final preparation of this work. Rashid, graduate students in civil engineering for their help at various stages of the work. RIVER SYSTEM VALLEYS, DELTAS, AND DEPOSITS Alluvial valley of the Lower Mississippi River.
Mississippi River flow and cross-sections of recent alluvial deposits in deltas in the Mississippi River Alluvial Valley. Communications and transport from Chalna to other parts of the province do not at present encourage the proper growth of trade and industry. The country feels the need to build highways, railways and waterways that connect all parts of the province to enable rapid economic growth.
This development should precede any other plan for the development of the province and the entire country. The apparent geological history and formations in the alluvial valley and deltaic regions of the Brahmaputra System, Rive r in East Pakistan are strikingly similar to the geological history and.
PAKI STAN
It is mainly bounded by India on three sides and by the Bay of Bengal to the south, as can be noted from Figure 2. The province of East Pakistan can be physically and geologically divided into three broad divisions: (a) The. gently rolling hilly portion of the southeastern region comprising the districts of Chittagong and Chitta gong Hill Tracts and a small portion of relatively high land in the northern portion. The alluvial valley of the rivers Brahmaputra and Ganges, and (c) The recent deltaic area in the south.
An elevation of 54 feet at the mouth of the alluvial plain, which is about 225 miles from the coast in the south, gives an average gradient of 3 inches per mile for the rivers (1). This very flat slope suggests that what now appears as an alluvial plain may have originally been a deltaic area. Due to the expansion of the delta formation into the sea, the coast moves further south and gradually this part became the alluvial plain of the rivers, Brahmaputra and Ganges.
EAST PAKISTAN
The highland, which covers parts of Mymensingh and Dacca counties, including the town of Dacca, is a ridge similar to the Crowley Ridge in the Mississippi alluvial plain. Due to the gradual deterioration of the immediate valley floor, the Ganges recently shifted its course eastward, possibly after the 15th century (4), and joined the mighty Brahmaputra River. The process of building alluvial valleys and expanding the deltas to the sea continues.
The Brahmaputra River rises in western Tibet and drains the vast basin north of the Himalayas. It is noteworthy that, while the river flows through the province of Assam, the river valley is bounded on the north by the Himalayas and on the south by smaller mountains with heights on the order of 1400 meters. A reference altitude of 17.5 meters (mean sea level) has been found for a point about 60 miles south of the common border of Assam and East.
This serves to confirm the indication of a flat gradient through much of the track through eastern Pakistan. The steep gradient of the river through much of its course can reasonably be taken as a strong indication of the ability to transport large quantities of detrital materials during a period of heavy outwash to the area occupied by East Pakistan.
TABLE 2
The Brahmaputra River was the main outlet for its large glacial water basin north of the Himalayas. It has evolved over the past few thousand years in response to the changing channels of the Mississippi River. In eastern Pakistan, the delta area occupies the majority of the area of the province and is formed mainly due to the displacement.
The peat deposits are located beneath the landward portion of the Chenier Plain in the Lower Mississippi Valley, at depths of more than 20 feet. In East Pakistan, peat deposits have recently been discovered at shallow depths in the sV{ampy area of Faridpur district, an area similar to the Chenier plain. Fisk (7) prepared an excellent and very valuable account of the geology of the alluvial valley of the Lower Mississippi River.
In 1881, deep drilling by the Mississippi River Commission revealed that the alluvium extended well below the maximum depth of the modern river. The width, depth and slope of the trenches varies accordingly depending on the hardness of the rock.
ALLUVIUM
LEGEND
MISSISSIPPI
RECENT
It is of particular interest to observe the vertical arrangement of the alluvium in the valley. A brief discussion of each of the two basic materials is presented under the following individual headings. The largest of the gravels are naturally concentrated in the lower part of the deposit.
The maximum particle size s has been reported to be over a foot in diameter r. The thickness of this part depends on the depth of the rooted valley that the gravel fills. The deposits that form the upper part of the alluvial section are sand, silt and clay.
These consist of marsh deposits and deltaic plains, which are varieties of non-gravel deposits. Further, this profile lies near the upper limit of the Mississippi River delta plain. Standard penetration samples of recent gravel deposits were secured and sealed in conventional glass jars.
The shear strength properties of the materials were determined by the unconsolidated rapid tri-axial shear tests. The purpose of soil constants is to determine the nature and general condition of the materials in question. It can be noted from the above graphs that the moisture content in the samples follows a general trend for all the boreholes.
The results of the standard penetration test carried out on the burial deposits at the three sites are shown in Table 5. The support capacity of friction piles depends on the characteristics of the surrounding materials. Illustrations of the use of the types of substructures in the Mississippi River delta plain are presented under the following subheadings.
This location was typical of the lower deltaic plain of the Mississippi River, in that the thickness of the non-. In this lower part of the deltaic plain the deposits appeared to be in a relatively unconsolidated state.
76 strength of the supporting medium and to cause settlement within the
Masters (11) presents the case of the Morganza Floodway where such short piles made of wood have been used successfully. The bearing capacity of the piles determined from load tests was found to be surprisingly close to the values predicted by Masters on the basis of his soil analysis. They show that certain points which can be considered to indicate the ultimate bearing capacity of the piles cannot be immediately established in the arithmetic scale plot.
It can also be noted that the retention point in the sand layers significantly increased the capacity of the piles. The immediate foundation medium of the embankment consisted of an upper layer of typical deltaic plain of the lower river, which. This material is similar to that found in the near surface 20 to 30 feet of the area from Baton Rouge to Lafayette, reported in Chapter IV of this dissertation.
The liquid limits of the sample were close to or slightly above IOO, whereas the plastic limits usually ranged in the twenties. In order to ensure the maximum possible strength increase during the minimum time, provision was made in the design that the surface of the natural soil or foundation medium was covered with a penetrating river sand. The remaining height of the rampart was formed from clay excavated from the river bank.
The maximum degree of consolidation of the embankment material was ensured due to the basic means of drainage provided in the project. Over time this pressure is released due to drainage of water from the soil mass. The applied stress is .. the available strength of the foundation media observing the pores.
For the area between the midpoint of the slope of the embankment and the slope of the planned slope, which is the most critical area, the connection. These circumstances dictate the urgent need for an extensive study of the general geology and condition of the foundation media in the province. Study of the history of gloicialization in the Northern Hemisphere during the Pleistocene Ice Age reveals the similarity between ice caps within the limits of the Mississippi River system to the north.
The dense sand layers in the upper parts of the grave-bearing deposits in the lower Mississippi Valley have been utilized and served as excellent foundation media for all kinds of heavy structures. Fisk, Harold N Geological Survey of the Lower Mississippi River Alluvial Valley", Mississippi River, Commission, Vicksburg, Mississippi, USA, 1944.
